We plan to examine the viral and cellular factors that affect the expression of Friend SFFV in a new cell culture system devised by Clarke et al. The conversion of normal bone marrow cells to an erythropoitein (Epo)-independent state after exposure to SFFV is detected by their ability to generate erythroid (benzidine-positive) colonies of 8 or more cells in plasma clot cultures within 48-72 hours in the absence of Epo. Factors that alter the release of virus from the IS cell line and other cell lines will be studied, as will the genotype of mice used as bone marrow donors. BIBLIOGRAPHIC REFERENCES: Steeves, R.A., and Grundke-Igbal, I.: Bacterial lipopolysaccharides as helper factors for Friend spleen focus forming virus in mice. J. Nat. Cancer Inst. 56: 541-546, 1976. Steeves, R.A., Blank, K.J., and Lilly, F.: Genetic control of Friend virus-transformed colony-forming cells by the FV-2 resistance gene in mice. In: Comparative Leukemia Research 1975, Eds. J. Clemmesen and D.S. Yohn, pp. 151-153 (Karger, Basel 1976).